Piston



March 7, 1939.

A. L, NELSON 2,149,862

PISTON Original Filed July 20, 1936 v 3 Sheets-Sheet 1 "H fly/2 INVENTOR.

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ATTORNEYS A. L. NELSON March 7, 1939.

PISTON Original Filed July 20, 1936 3 Sheets-Sheet 2 INVENTOR. BY Awe/$0 M, 2a); ma

ATTORNEYS,

A. L. NELSON Ma h 7, 1939.

PISTON Original Filed July 20, 1936 3 Sheets-Sheet 3 v ii 1D.

IN VENT OR.

BY 40414 L/l/e/sw;

Patented Mar. 7, 1939 UNITED STATES PISTON Adolph L. Nelson, Detroit, Mich.

Application July 20, 1936, Serial No. 91,458

Renewed January 13, 1938 19 Claims.

This invention relates to an improved piston adapted for internal combustion engines.

It is-particularly concerned with the provision of a piston of light-weight material, such as an alloy of aluminum. Materials of this kind while having advantages of lightness and high thermal conductivity, have higher rates of thermal expansion than the material of the engine cylinders, which is usually cast-iron, and the present invention provides means to compensate for this excess expansion of the light-weight piston.

A main object of the invention is to provide a piston in which the head and skirt are united as a single, homogeneous piece of light-weight piston material having sufiicientstrength and stability to operate satisfactorily in an engine without additional supporting members, but having thermal plates of material with a coefiicient of thermal expansion lower than the corresponding coeflicient of the skirt "material.

A feature of the invention resides in the fact that part of the piston metal forms webs set back from the normal periphery of the piston. Short thermal plates are secured along the inner sides of two or more of these webs and cooperate with the webs to form bimetallic members which tend to change shape under an increase of temperature. The skirt is originally given an ovalexterior shape, and the bimetallic members cause the skirt to change shape under an increase of temperature insuch a way as to compensate for the excess expansion of the skirt material.

Particular advantages of using short thermal plates as disclosed herein are savings in weight and cost, and the fact that they do not interfere with the design-of the load-carrying piers between the head and the piston pin bosses.

Other objects and advantages of the invention will become evident as the description proceeds.

While particular forms of the invention are disclosed herein for purposes of illustration, it should be understood that various changes may be made in the structure and arrangement without departing from the spirit of the invention herein set forth and claimed.

In the drawings:

Fig. 1 is a perspective view of a piston embodying the invention.

Fig. 2 is an elevation, partly in section of the piston shown in Fig. 1.

Fig. 3 is a section on line 3-3 of Fig. 2.

Fig. 4 is a section on line 4-4 of Fig. 3.

Fig. 5 isa perspective of one of the inserts used in the piston of Figs. 1 to 4.

Fig. 6 is an enlarged section showing diagrammatically the changes effected by heat.

Fig. '7 is a view similar to Fig. 3 but showing a modification.

Fig. 8 is a perspective view of one of the inserts 5 used in Fig. 7.

Fig. 9 is a section on line 9-9 of Fig. '7.

Fig. 10 is a view similar to Fig. 3 but showing a further modification.

Fig. 11 is a sectionon line li-li of Fig. 10. 10

Fig. 12 is a cross-section showing another modification.

Fig. 13 is a section on line l3-l3 of Fig. 3.

The drawings show a piston having a head ill,

a skirt II, and piston pin bosses l2, the main 15 loads between the head and bosses being carried by piers l3. Each pier i3 is of approximately U-shape when viewed in horizontal cross-section, and includes an outer web l4 connecting with the outer end of the pin boss, side ribs I5 and a central rib IS.

The skirt includes curved thrust walls l1, which carry the bearing faces l8 and iii. The lateral edges of faces l8 and 19 are defined by curved lines 20 which separate the bearing faces from areas 2| which are relieved, that is, set back from the normal surface of the skirt.

Flat webs 22 connect the lateral edges of the curved thrust walls I! with the head, the piers l3, and the piston pin bosses l2. The webs 22 are 3 quite wide, being illustrated as substantially as wide as the outer diameter of the pin bosses. Short stiffening ribs 23 extendoutwardly from the lower part of each pin boss toward bearing face i8.

Curved bands 24 connect the lower ends of the thrust walls I I and cooperate with said walls I! to form a substantially complete annulus at the lower end of the skirt. Each band 24 is separated from its associated pin boss by an opening 25 which is wider and longer toward the face l9 than toward the face l8.

Each of the thrust walls is separated from the head -by a slot 26, and a slot 21 extends through bearing face l8 in a substantially vertical direction. All of the parts thus far described are formed of a suitable light-weight piston material, such as an alloy of aluminum, and constitute a piston that has been giving satisfactory service as regu- 5o lar equipment in one of the leading automobiles.

I have found that the operation of this piston can be improved-by providing it with thermal plates which cooperate with parts of the piston to form bimetallic members. These thermal plates material composing the other parts of the piston.

In the piston illustrated in Figs. 1 to 4 the thermal plates 28 are small rectangular plates cut from sheet steel. Each thermal plate is formed with two openings 29, and along its vertical edges with projections separated by recesses This piston can be economically manufactured in metal molds. In doing so the inserts are positioned in the molds by hooking their openings 29 over pins carried on the cores, before the cores are inserted in the molds. Pins mounted on the mold body press the inserts firmly against the cores, leaving openings 32 in webs 22 of the finished casting. When the molten piston metal is poured into the mold it flows around the projections 30, causing those at one end of the insert to be partly embedded in the inner wall of one of the curved portions ll, while those at the other end of the insert are partly embedded in one of the pin bosses.

The thermal plates serve no function in uniting the other parts of the piston, and hence the embedding oi the ends of these plates is merely to hold them in place. The projections 3d are present only to increase the thickness of skirt material opposite the recesses 3!, as clearly shown in the lower part of Fig. 4.

In finishing the piston the head is given a sufdciently smaller size than the skirt to prevent its contacting with the wall of the cylinder at any time, and the skirt is given an oval exterior shape, the minor axis AB of the oval coinciding with the axis of the piston pin openings, and the major axis CD lying at right angles thereto. The piston is fitted to its cylinder with a suitable working clearance at the ends of axis CD, there being excess clearance at the ends of axis AB.

Accurate expansion tests have been made on these pistons by heating samples in an electric furnace and by measuring the change in size at various points on the skirt. By this method it has been found that as the piston temperature rises the parts make the relative movements shown in dotted lines in Fig. 6, which movements are, I believe, due to the following action.

After the piston is originally cast and the metals are cooling, the aluminum of the webs 22 shrinks faster than the steel of the plates 2% on account of the larger coeflicient of thermal expansion and contraction of the aluminum. The result is that at room temperatures a web 22 and its cooperating plate 28 form a bimetallic member in which the aluminum web is under tension and the member is slightly bent with its convex side toward the inside of the piston, as illustrated in full lines of Fig. 6. When the piston is subsequently heated the heat relieves the tension' in the aluminum, permitting the member to resume its fiat shape.

The head, being the hottest part of the piston in engine operation, expands more rapidly than the skirt, and since the bosses are rigidly braced to the head they are carried outwardly by the expansion of the head.

The bimetallic members located. betweenthe pin bosses and the slotted thrust face 58 can be considered to combine with the halves of this 'thrustface to form members that are substantially L-shapedwhen viewed in cross section as in The upper end of each of.these bimetallic members is rigid with its associated pin boss, and hence the rise in temperature, which expands the web 22, tends to rotate the L-shaped' member about the point of junction of its two branches, and to simultaneously translate the lower end of the L-shape member toward the center-line C-D. This action reduces the size of the skirt adjacent line CD and reduces the width of slot 21 independent of any pressure of the skirt against the cylinder wall.

The bimetallic members between the pin bosses and the thrust face Hi can also be considered to form L-shaped members, which tend to rotate about their apexes and to carry inwardly the parts of the skirt adjacent line C-D.

In the piston illustrated in Figs. 1 to 4 the skirt is stiffened on the slotted side because of the presence of the ribs 23 and to the reduced extent of the openings 25, with the result that the inward deflection on the thrust face 18 is substantially equal to that on the thrust face 19, whereas without this extra stifiening the deflection of thrust face [8 would be greater.

I have found that in this piston the expansion on the diameter AB is actually less than the rate of aluminum, apparently due to the action of the bimetallic members in closing the slot 21.

It should be noted that in this piston the bimetallic members are spaced from the periphery of the skirt except at their outer ends, hence'their bending action does not introduce any localized deformations in the cylinder-contacting portion of the skirt wall, but causes a uniform bending of the skirt as illustrated diagrammatically in Fig. 6.

The amount of compensatory bending induced in the skirt can be varied by changing the length of the thermal plates, since the shorter the plates the less bending action provided the rest of the piston remains unchanged. Also, forming the plates from materials having diflerent coefiicients of expansion will vary the amount of bending. In some piston designs it may be preferable to use very short thermal plates, and to compensate for shortening the plates by forming them of some material that has less expansion than ordinary steel, as for example a nickel-steel. Fig. 4 of my Patent No. 1,681,709 shows how the coefilcient of expansion of iron-nickel alloys varies with the nickel content, and that table will assist in selecting an insert material for a particular installation.

The amount ofbending can also be varied by changing the relative cross-sectional sizes of the webs 22 and the thermal plates. Thus if the webs 22 are weaker in relation to the thermal plates, they will cause less bending of the thermal plates and hence less bending of the skirt.

It should be understood that the dotted lines of Fig. 6 merely show in a general way and on a. greatly exaggerated scale the relative movements of the parts. In an actual piston of approximately three inch size these movements are measured in thousandths of an inch.

The thermal plates serve solely as compression members which prevent the webs 22 from contracting as they normally would, thus permitting the bending movements as described above.

Figs. 7 to 9 illustrate a modified form of thertrial plate 33. In this form a single aperture 36 opens in the form of a keyhole toward one side of the insert. This aperture 36 has a snap frictional fit over a pin carried by the core to hold the ly the same width and 11 the short thermal plates extent than the "webs 22.

in which the curved thrust walls are connected they may be retained in any convenient manner. Thus in the modification illustrated in Figs. 31 are secured to the webs 22 by buttons 38 of the web material passing through openings of the inserts. The plates have straight vertical edges widened to form feet 39, one of which merely abuts against ashoulder formed on web 22, while the other one abuts against the skirt wall. This abutting arrangement of the parts is made possible by the shrunk-on character of the bimetallic members as explained above. "ihese figures also show that the webs 22 need not extend at right angles to the axis of the.pin bosses, but may be disposed at an angle thereto.

Fig. 12 shows a lie members extend from the pin bosses toward the thrust face l8 but are absent on the other side of the piston.

Instead of a slot extending entirely through a thrust face, a slot may extend part way down into a thrust face, as illustrated at 40 in Fig. '1, or a similar slot may extend part Way up into a thrust face. In other cases the slot, may be closed at both ends as shown at. in Fig. 10, or it may be omitted altogether.

In each of the forms illustrated the thermal plates are shown as slightly narrower in vertical Although this is the preferred arrangemen it need not be adhered to, as will be understood by those skilled in the art.

I claim:

1. A piston comprising a head, piston pin.

bosses, a skirt having an oval shape with the minor axis of the oval coinciding with the axis of the piston pin bosses, curved thrust walls located on opposite sides of the skirt and separated from the head by slots, the head, pin bosses, and thrust walls being of light-weight piston material, and bimetallic elements extending from at least one thrust face to the piston pin bosses, each bimetallic element consisting of an innermost member and an outer-most member, said innermost member being-a plate of material having a lower rate of thermal expansion than the material of the thrust walls and located between a pin boss and a thrust wall, said outermost member being a web of material similar to that'of the thrust walls and being set back from the periphery of the skirt and being shrunk on the innermost member, said bimetallic elements being adapted to bend under an increase of temperature to change the shape of the skirt and compensate for the excess expansion of the skirt.

2. A piston as recited in claim 1 in which a slot extends vertically in one of the thrust walls.

3, A piston as recited in claim 1 in which said outermost member is substantially as wide as the outer diameter of the pin boss and the said innermost member is of substantially the same width as the outermost member.

,4. A piston as recited in claim 1 slot extends vertically in one of the thrust walls and in which said outermost member is substantially as wide as the outer diameter of the pin boss and the said innermost member is of substantialas the outermost member.

5. A piston as recited in claim 1 in which the curved thrust walls are connected together by arcuate bands extending under the pin bosses, each arcuate bandbeing separated from a pin boss by an opening.

6. A piston as recited in claim 1 in which a slot extends vertically in. one of the thrust walls and modification in which bimetalcurved thrust walls ,.under the pin bosses, in which a connected together together by arcuate bands extending under the pin bosses, each arcuate band being separated from a pin boss by an opening, the openings extending across a vertical plane passing through the center line of the pin bosses, each opening extending farther toward the thrust face and being wider on the side of said plane away from the slotted thrust wall than on the side toward the slotted thrust face.

'7. Apiston as recited in claim 1 in which a slot extends vertically in one of the thrust walls and in which the curved thrust walls, are connected together by arcuate bands extending under the pin bosses, each arcuate band being separated from a pin boss by'an opening, the openings ex? tending across a vertical plane passing through the center line of the pin bosses, each opening extending farther toward the thrust face and being wider on the side of said plane away from the slotted thrust wall than on the side toward the slotted thrust face, and a tapered rib extending along a web from the lower part of each pin boss toward the slotted thrust face.

8. A piston comprising a head, piston. pin bosses, a skirt having an oval shape with the minor axis of the oval coinciding with the axis of the piston pin bosses, curved thrust walls located on opposite sides of the skirt and separated froni the head by slots, webs extending from the pin bosses to the thrust walls, the head, pln' bosses, thrust walls and webs being of light-weight piston material, and four plates of material having a lower rate of thermal expansion than the material of the webs, each of said plates extending along the inner wall of one of said webs, the plates cooperating with the webs to form bimetallic thermostatic elements which bend under an increase in temperature to change the shape of the skirt and compensate for.

the excess expansion of the skirt.

9. A piston as recited in claim 8 in which a slot extends vertically in one of the thrust walls.

10. A piston as recited in claim '8 in which said webs are substantially as wide as the outer diameter of the pin bosses and the said plates are slightly narrower than the webs.

11. A piston as recited in claim 8 in which a slot extends vertically in one of the thrust walls, and in which said webs are substantially as wide as the outer diameter of the pin bosses and the said plates'are slightly narrower than the webs.

12. A piston as recited in claim 8 in which the are connected together by arcuate bands extending under the pin bosses, each arcuate band being separated from apin boss by an opening.

13. A piston as recited in claim 8 in which separated from a pin openings extending across a vertical plane passing through the center line of the pin bosses, each opening extending farthertoward the thrust face and being wider on the side of said plane away from the slotted thrust wall than on the side toward the slotted thrust face.

14. A piston as recited in claim 8 in which a slot extends vertically in one of the thrust walls and in which the curved thrust walls are by arcuate bands extending under the pin bosses, each arcuate band being separated froma pin boss by an opening, the

- aleiwi 3 1 openings extending across a vertical plane passing through the center line of the pin bosses, each opening extending farther toward the thrust face and being wider on the side of said plane away from the slotted thrust wall than on the side toward the slotted thrust face, and a tapered rib extending along a web from the lower part of each pin boss toward-the slotted thrust face.

15. A piston comprising a head, piston pin bosses, a skirt having an oval shape with the minor axis of the oval coinciding with the axis of the piston pin bosses, curved thrust walls located on opposite sides of the skirt and separated from the head by slots, each thrust wall carrying a thrust face bordered by relieved areas, the sides of the thrust faces being defined by lines which are curved convexly with relation to the thrust face and which converge toward the top of the thrust face, webs extending from the pin bosses to the thrust walls, the head, pin bosses, thrust walls and webs being of lightweight piston material, and four plates of material having a lower rate of thermal expansion than the material of the webs, each of said plates extending along the inner wall of'one of said webs, the plates cooperating with the webs to form bimetallic thermostatic elements which bend under an increase in temperature to change the shape of the skirt and compensate for the excess expansion of the skirt. i

- 16. A piston, as recited in claim 15 in which I a slot extends vertically in one of the thrust walls.

17. A piston as recited in claim 15 in which said webs are substantially as wide as the outer diameter of the pin bosses and the said plates are slightly narrower than the webs.

18. A piston as recited in claim 15 in which a slot extends vertically in one of the thrust walls, and in which said webs are substantially as wide as the outer diameter of the pin bosses and the said plates are slightly narrower than the webs.

19. A piston comprising a head, piston pin bosses, a skirt having an oval shape with the minor axis of the oval coinciding with the axis of the piston pin bosses, curved thrust walls located on opposite sides of the skirt and separated from the head by slots webs extending chordally of the piston from one thrust wall to the other, the head, pin bosses, thrust walls and webs being of light-weight piston material, and plates of material having a lower rate of thermal expansion than the material of the webs, each of said plates extending along the inner wall of one of said webs, each plate having one of its ends located adjacent a thrust wall and its opposite end located adjacent a pin boss, the plates cooperating with the webs to form-bimetallic thermostatic elements which bend under an increase in temperatur'eto change the shape of the skirt and compensate for the excessexpansion of the skirt.

I v ADOLPH L. NELSON. 

